Emerging fiber optics and integrated optics technologies promise to have a large impact on communications and information processing technologies. Information transfer by means of fiber optics data busses offers many advantages over competing systems. These advantages are realizable since this field has been strongly stimulated by the recent development of glass fiber waveguides having an attenuation of 20 dB/km a few years ago and more recently of glasses which reduced this value to 2.1 dB/km. This has led to an examination of systems using light emitting diodes, multi-mode waveguide bundles, and photodetector components. Systems using optical fiber transmission lines have the advantage of being free from electromagnetic interference and crosstalk; they have greater security since there is no signal leakage; they have no electrical ground or short circuit problems; they have large bandwidth for their size and weight; they have small size, light weight and great flexibility by comparison to copper wire cable harnesses; they have high temperature tolerances and safety in combustible areas; they have high tensile strength and do not require the use of copper which is becoming a strategic material in short supply. When coupled with the use of integrated optics such systems have the advantage of greater switching speed, and a larger number of carrier frequencies for multiplexing resulting from their greater bandwidth.
The waveguide used in such systems may be either an optical fiber waveguide which contains either a step or a graded change in the refractive index such that light propogating within the fiber is guided in the core region, or it may be a channel waveguide formed in a semiconductor chip to provide a similar change in refractive index. One of the problems in such optical waveguide communication or data links is how to introduce an optical signal onto the line at one point and then to read it off the line at some distant point. One way this can be accomplished in the case of a waveguide consisting of optical fiber bundles is to split off a few of the optical fibers at each desired terminal point and connect them to a discrete light emitter or detector. However, if both transmission and reception are required at a given terminal, either two sub-bundles must be split off or a coupling and/or mixing block must somehow be provided to allow light signals to flow in both directions from the terminal.